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Biogenic Opal in Ohio Soils¹

ABSTRACT

Soils in the eastern extremity of the Prairie Peninsula have developed during most of their history under forested conditions. Biogenic opal isolated from the 50-20µ fraction of 55 profiles representing different topo- and biosequences in western Ohio indicates that most Mollisols (Brunizems) of this area have similar total opal accumulations and depth-distributions as their Alfisol (Gray-Brown Podzolic) analogues. The prairie influence on both groups of soils has been calculated to be about 150 to 500 yars. Opal depth distributions decrease from 0.1 to 0.3% in surficial horizons to <0.03% below 50 cm. No evident differences in opal concentration occur among toposequence members. Opal isolated from leaves of deciduous trees common to Ohio is optically isotropic, has a specific gravity <2.3, and a refractive index between 1.44 and 1.48. With few exceptions, <2% of the forest opal occurs in the 50-20µ size range with >75% in finer fractions. Scanning electron microscopy, reveals that opal in >20µ sizes isolated from tree leaves is sufficiently distinct morphologically that it would not normally be confused with that of grass origin. Opaline constituents of forest origin are generally 1 to 2µ fragile incrustations of the cell walls and seldom form solid rectangular or polyhedral structures characteristic of opalized cells of grass origin.

¹ Contribution from the Department of Agronomy, Ohio Agr. Res. & Develop. Center, State Project 371, Journal Paper no. 14-71. Presented before Div. S-9, Soil Science Society of America, Tucson, Ariz., Aug. 24, 1970.

² Professor and Technical Assistant of Agronomy, respectively, Ohio Agr. Res. & Develop. Center, Wooster and Ohio State Univ., Columbus 43210.

Received for publication February 25, 1971. Accepted for publication August 25, 1971.